allow '--help' to print subcomands info

Currently, we only allow $manila help subcomand to print
subcomands info, if we are using $manila subcomand --help,
it will print all the subcomands info, that is not what we
want, this change is to allow '--help' to print subcomands
info. finally we can see the following comands are equals:

$ manila --help list
$ manila list --help
$ manila help list

Change-Id: I926defd6c5d3b8d87b038e2c63db694b40a88359
Close-bug: #1771097

manilaclient/shell.py

@@ -465,6 +465,12 @@ class OpenStackManilaShell(object):
         self.parser = self.get_subcommand_parser(
             os_api_version.get_major_version())
 
+        if argv and len(argv) > 1 and '--help' in argv:
+            argv = [x for x in argv if x != '--help']
+            if argv[0] in self.subcommands:
+                self.subcommands[argv[0]].print_help()
+                return False
+
         if options.help or not argv:
             self.parser.print_help()
             return False

manilaclient/tests/unit/test_shell.py

@@ -290,12 +290,13 @@ class OpenstackManilaShellTest(utils.TestCase):
     def test_help_unknown_command(self):
         self.assertRaises(exceptions.CommandError, self.shell, 'help foofoo')
 
-    def test_help_on_subcommand(self):
+    @ddt.data('list --help', '--help list', 'help list')
+    def test_help_on_subcommand(self, cmd):
         required = [
             '.*?^usage: manila list',
             '.*?(?m)^List NAS shares with filters.',
         ]
-        help_text = self.shell('help list')
+        help_text = self.shell(cmd)
         for r in required:
             self.assertThat(help_text,
                             matchers.MatchesRegex(r, re.DOTALL | re.MULTILINE))

releasenotes/notes/add-dash-dash-help-subcomand-ad4226454aa07bc6.yaml

@@ -0,0 +1,4 @@
+---
+fixes:
+  - Allow --help to print subcomands info. e.g.
+    ``$ manila create --help``